The present invention relates steganography and more particularly to the digital watermarks.
The technology for applying digital watermarks to images and to other types of data is well developed. For example see issued U.S. Pat. No. 5,748,783, issued U.S. Pat. No. 5,768,426 issued U.S. Pat. No. 5,822,435 and the references cited in these patents. Also various commercially available products (such as the widely used image editing program Photoshop(trademark) marketed by Adobe Corporation) have image watermarking capability. There are many other patents and much technical literature available relating to the application of digital watermarks to images and to other types of data.
Co-pending application Ser. No. 09/553,084 describes a technique of color adaptive watermarking. With the technique described in application Ser. No. 09/553,084 a change in an image attribute such as luminance (or chrominance) is mapped to a change in color components such that the change is less visible application Ser. No. 09/553,084 describes the xe2x80x9cscale to blackxe2x80x9d and the xe2x80x9cscale to whitexe2x80x9d techniques for applying watermarks. By using the scale to white method for colors with a high yellow content such as yellow, red and green, and by using the scale to black for blue, cyan and magenta a watermark with a lower visibility and the same detect ability can be embedded in an image.
It is known that when an image is printed on a standard offset press, the relationship between the digital value of a color and the amount of ink actually applied by the press is not linear. FIG. 1 illustrates the dot gain curve for a typical standard offset printing press. The horizontal axis gives a digital value of a color and the vertical axis indicates the amount of ink actually transferred by the press. The shape of the dot gain curve of offset printing presses is well known.
As a result of the dot gain curve illustrated in FIG. 1, when an image containing a watermark is printed on an offset press, a watermark signal in the shadows (i.e. in an area with more ink) is reduced and a watermark signal in the highlights (i.e. in an area with less ink) is amplified. Note that the slope of the dot gain curve is different in the shadow area and in the highlight area. Thus, the same amount of change in color value produces a different amount of change in the ink applied in the two different areas. The present invention provides a technique which insures that a watermark signal is preserved in an printed image as accurately as possible not withstanding the fact that the dot gain curve of the printing press is not linear.
With the present invention, the image data is first modified in accordance with the forward dot gain curve of a printing press, next the watermark xe2x80x9ctweakxe2x80x9d values (i.e. the watermark change values) are calculated for this modified image data. The calculated xe2x80x9ctweakxe2x80x9d values are then modified in accordance with the backward dot gain curve of the printing press. The modified tweak values are then added to the original image data values to produce a watermarked image. The watermark image is then printed on the printing press. The result is that the xe2x80x9ceffectivexe2x80x9d tweak on printed paper is not materially affected by the dot gain curve of the printing press.